Optical spans, such as optical fibers, may be used to transmit data-carrying signals from a sender to a receiver. When transmitting data over an optical span, the phase and polarization of the data-carrying signal can be altered due to unpredictable events. For phase-modulated signals, in which the phase of the received wave determines the value of the received data, unpredictable changes to the phase of the signals during transmission can cause erroneous information to be received by the receiver. If polarization-multiplexing of two phase-modulated signals over the same carrier is used, unpredictable changes to the polarization of the signals during transmission can cause erroneous information to be received by the receiver.